BBC1's Idris Elba detective drama Luther comfortably won the 9pm hour again last night, Tuesday 18 May, but has shed nearly 30% of its audience in three weeks.

Luther attracted 4.051 million viewers and a 17.9% audience share on BBC1.

This was down from BBC1 figures of 5.313 million (21.4%) last Tuesday and 5.646 million (23.8%) for the show's debut on 4 May. Luther's audience share is also declining – down nearly 25% compared with the show's debut two weeks ago.

Yesterday normal service resumed in the 8pm hour on BBC1, with Holby City attracting 5.706 million (26.7%).

Some of the audience decline last night can be attributed to the improved weather and fewer people watching TV compared to last Tuesday – at 22.6 million, some 2.2 million less in the 9pm hour overall.

Last Tuesday's political drama with the formation of the Liberal Democrat-Conservative coalition also boosted the audience for BBC News Channel and Sky News enormously in the 9pm hour, with an average just over 1.3 million viewers for the two networks. This is probably about 10 times their normal combined audience in the slot.

The week-on-week decline in the overall TV audience in the 9pm hour was 9%, compared with nearly 24% for Luther.

However, Luther's decline does not appear to have benefited the other main channels, with only Channel Five's CSI posting a gain in viewers and share compared with last Tuesday (2.003 million/8.9% v 1.908 million/7.7%).